1. Field of the Invention
This invention relates to a brush seal assembly adapted to effect a seal between a first component and a surface of a second component which is relatively movable with respect to the first component.
2. Description of the Related Art
It is frequently necessary to effect a fluid-tight seal between two relatively movable components, such as between a rotatable shaft and a housing having a bore through which the shaft extends. Innumerable seal designs have been proposed for this purpose and a machine designer has to select the most appropriate type of seal, having regard to the likely operating conditions to be encountered in service.
For the case of a shaft extending through a bore in a housing and which shaft rotates at a relatively high speed in the presence of pressurised hot gases, such as may be encountered in a gas turbine or a jet engine, various designs of brush seal have been developed in an attempt to overcome the unsatisfactory performance of simpler seals having for example a synthetic elastomeric annular sealing member mounted in the bore of the housing and rubbing on the surface of the shaft. In a brush seal, a plurality of fine bristles (such as of bronze or stainless steel wire) are held in a carrier mounted on the housing, with the tips of the bristles wiping the surface of the shaft so as to effect a seal thereagainst.
In a typical brush seal intended for use with a pressure difference thereacross, the bristles are supported against deflection in a direction parallel to the shaft axis for the greater part of the length of the bristles by means of an annular backing plate on the low-pressure side of the seal, with the tips of the bristles projecting radially beyond the backing plate to wipe against the shaft surface to be sealed. If it can be expected that the shaft is likely to perform relatively large radial excursions--for instance, as may occur when an engine rotor is being run up to operating speed--the clearance between the backing plate and the shaft must be increased, so leading to a greater unsupported length of bristle, projecting beyond the backing plate. If then the seal is subjected to a relatively large pressure difference, the unsupported bristle tips projecting beyond the backing plate will tend to deflect towards the low pressure side and so lift clear of the shaft, and the sealing effect will be lost. If the bristles are made stiffer (for example by increasing the diameter of each bristle) the bristles may be too stiff to give an adequate sealing function able to accommodate minor radial excursions between the shaft and the housing; also rapid wear of the shaft may be expected as a consequence of the relatively stiff bristles wiping the shaft.
In addition to the above problem, it is possible for a brush seal to be subjected to an abnormal pressure surge where the side of the seal normally subjected to high pressure is suddenly subjected to a lower pressure than is present on the other side of the seal. If this occurs, the seal can be destroyed. It will be appreciated that it is in general not possible to employ a second backing plate on the high pressure side of the seal, since the pressure of the two backing plates is likely to interfere with the free movement of the bristles.
It is a principal aim of the present invention to provide a brush seal assembly adapted to effect a seal between two relatively movable components and which is able to accommodate relatively large excursions of one component with respect to the other and out of the normal line of movement between the two components, and yet which seal assembly is still able to achieve a good sealing effect notwithstanding a significant pressure difference thereacross.